BACKGROUND AND PURPOSEIn mammalian cells, the anti-parasitic drug ivermectin is known as a positive allosteric modulator of the ATP-activated ion channel P2X4 and is used to discriminate between P2X4-and P2X7-mediated cellular responses. In this paper we provide evidence that the reported isoform selectivity of ivermectin is a species-specific phenomenon. EXPERIMENTAL APPROACHComplementary electrophysiological and fluorometric methods were applied to evaluate the effect of ivermectin on recombinantly expressed and on native P2X7 receptors. A biophysical characterization of ionic currents and of the pore dilation properties is provided. KEY RESULTSUnexpectedly, ivermectin potentiated currents in human monocyte-derived macrophages that endogenously express hP2X7 receptors. Likewise, currents and [Ca 2+ ]i influx through recombinant human (hP2X7) receptors were potently enhanced by ivermectin at submaximal or saturating ATP concentrations. Since intracellular ivermectin did not mimic or prevent its activity when applied to the bath solution, the binding site of ivermectin on hP2X7 receptors appears to be accessible from the extracellular side. In contrast to currents through P2X4 receptors, ivermectin did not cause a delay in hP2X7 current decay upon ATP removal. Interestingly, NMDG + permeability and Yo-Pro-1 uptake were not affected by ivermectin. On rat or mouse P2X7 receptors, ivermectin was only poorly effective, suggesting a species-specific mode of action. CONCLUSIONS AND IMPLICATIONSThe data indicate a previously unrecognized species-specific modulation of human P2X7 receptors by ivermectin that should be considered when using this cell-biological tool in human cells and tissues. AZ 10606120, N-[2-[[2-[(2-hydroxyethyl)

Extracellular ATP activates receptors such as P2X ligand-gated ion channels, but it also chelates divalent cations. Nörenberg et al. find that experimental conditions designed to measure P2X7 activity also activate TRPM7 channels, by relieving inhibition by extracellular divalent cations, in HEK293 and rat C6 glioma cells.

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